Typically the body is moved from the impact position to the retracted position by a hydraulic ram, against the force of gravity and/or against another force. Such a device is described in EP 708864A. In the device of this specification hydraulic rams lift a body, against the force of gravity and against the force provided by two elastic ropes, to the retracted position, maximally removed from the impact position. When the retracted position is reached a magnetic sensor triggers the disconnection of the feed of hydraulic fluid to the cylinders of the rams and the body accelerates to the impact position, under its own weight and the spring force.
This method is effective but there is loss arising from the hydraulic resistance of the hydraulic rams, as the body moves from the retracted position to the impact position. Although the cylinders are no longer subject to pressurised fluid they do not easily vent their contents rapidly as the body falls. As well as causing loss of power the heat generated in the hydraulic cylinders and associated control circuits can be considerable and could lead to leakage, and early failure.